This invention relates to a magnetic toner developing device utilizing a single component toner for developing images on a photoconductive sheet master. More particularly, it concerns an electrically isolated guide for supporting a sheet master during development to minimize the clamping effect on the sheet, caused by the electrostatic field between the developer brush and the guide, and to permit uniform advancement of the sheet through a development zone.
In the lithographic printing and duplicating field various types of planographic printing plates or masters are used for reproducing duplicated copies. Some masters receive a direct image from a typewriter or the like, while others have a sensitized surface for receiving a photographic image and still others have a photoconductive surface for receiving an electrostatic image. Conventionally, a photoconductive master sheet is uniformly charged by a corona charging means and exposed to a document to be copied via an optical projection system to produce an electrostatic latent image on its surface. The latent image is subsequently developed with a magnetic toner to produce a visible image. The master may then be treated with a conversion solution to provide a lithographic surface for producing offset lithographic copies, all in a conventional manner.
In the development of the latent images on the sheet masters, in two-component systems a "soft" developer brush is used and the charged master passed over its periphery. No backup plate is required. In single component toner systems, there is no triboelectric charge between the toner particles and a "thin" developer brush is required, with close spacing and tolerances being used in the development zone. Thus a guide plate is used to support the charged master which is passed under the developer brush. The terms soft and thin are of course relative. In the single component system the developer brush has on the order of one sixty fourth to one eighth inch of toner, whereas in the conventional two component system it is one quarter inch or more. With the single component magnetic toner, difficulty has been experienced in advancing the sheets through the developing station in the smooth, uniform and continuous manner needed for the production of high quality imaged sheets.
The single component developing station includes a magnetic developer brush of conventional construction, but of rigid configuration for applying toner to the sheet and a grounded metallic guide for supporting the sheet in its travel through the developing station. In a photoconductive master system, the latent image results in the insulated photoconductor sheet being charged. The charged photoconductive sheets tend to "hang-up" due to the clamping effect of the capacitor formed by the charged photoconductive layer as one plate, the insulated base sheet as the dielectric, and the conductive guide, in contact with the other side of the sheet, as the other plate. This clamping effect, which seriously impairs movement of the sheet, is particularly pronounced when developing sheets with large, dense image areas i.e., sheets having a great deal of surface charge, especially in low relative humidity environments.
Because of the thinness of the sheet, the electric field is fairly strong and results in the above-mentioned clamping effect, especially in heavily charged areas.
Therefore, it is desirable and an object of the present invention to reduce and/or substantially eliminate the undesirable clamping effect created by a charged master sheet passing through a development zone.